1. Field of the Invention
The present invention relates to a method for preparing a composite ion exchange membrane, and in particular, for making a composite ion exchange membranes for use in solid polymer electrolyte fuel cells.
2. Description of the Related Art
Ion exchange membranes are principal components of electrochemical cells such as solid polymer electrolyte fuel cells, chlor-alkali electrolysis cells, and batteries. Ion exchange membranes are also employed in diffusion dialysis, electrodialysis, pervaporation, and vapor permeation applications.
Anion, cation, and amphoteric ion exchange membranes are known.
Ion exchange membranes may comprise dense polymer films. For example, Nafion® membranes are commercially available dense film perfluorosulfonic acid (PFSA) ion exchange membranes suitable for use in solid polymer electrolyte fuel cells and chlor-alkali electrolysis cells. The polymeric compositions comprising substituted α,β,β-trifluorostyrene monomers disclosed in U.S. Pat. No. 5,422,411 may also be used to prepare such dense films for ion exchange membranes.
Currently available dense film ion exchange membranes suffer certain practical limitations for use in electrochemical cells such as fuel cells, such as for example cost and thickness.
For ease of handling, for example in the preparation of membrane electrode assemblies (MEA) for use in fuel cells, the mechanical strength of the membrane in the dry state and hydrated state is important. In electrochemical applications, such as electrolytic cells and fuel cells, the dimensional stability of the membrane during operation is also important. Further, to improve their performance, it is generally desirable to reduce the membrane thickness and to decrease the equivalent weight of the membrane electrolyte, both of which tend to decrease both the mechanical strength and the dimensional stability in the hydrated state.
One approach for improving mechanical strength and dimensional stability relative to dense film ion exchange membranes is through the use of a porous reinforcing support material. For example, an unsupported membrane can be preformed and then laminated to the reinforcing support, or a dense film may be formed directly on a surface of the reinforcing support. The reinforcing support is typically selected so that it imparts some mechanical strength and dimensional stability relative to the dense film ion exchange membrane. Composite membranes (discussed below) have also been laminated with reinforcing supports to form reinforced membranes.
Laminating or otherwise combining a reinforcing support with a dense film membrane or a composite membrane, while increasing mechanical strength and dimensional stability, is however not totally beneficial. One reason is that the reinforcing support tends to defeat the purpose of a thin membrane by increasing the overall thickness. Another reason, which also leads to reduced ionic conductivity, is due to the “shadowing” effect of the reinforcing support. The shortest path for an ion through a membrane is a perpendicular path from one surface to the other surface. Reinforcing supports are typically made from materials that are not ion-conductive. Those parts of the reinforced ion exchange membrane where an ion cannot travel perpendicularly across the membrane, but must take a circuitous route around the reinforcing support, are “shadowed” areas. The presence of shadowed areas in the reinforced membrane reduces the effective area of the membrane that actively conducts ions, thereby decreasing the effective ionic conductivity of the membrane.
Another approach for improving mechanical strength and dimensional stability in ion exchange membranes is to impregnate an ion-conductive material into a porous substrate material to form a composite membrane. Such composite ion exchange membranes prepared by impregnating a commercially-available microporous polytetra-fluoroethylene (ePTFE) film (Gore-Tex®) with Nafion®, have been described in the Journal of the Electrochemical Society, Vol. 132, pp. 514-515 (1985). The major goal in the study was to develop a composite membrane with the desirable features of Nafion®, but which could be produced at a low cost. Similarly, U.S. Pat. No. 5,547,551, U.S. Pat. No. 5,599,614 and U.S. Pat. No. 5,635,041 describe composite membranes comprising microporous expanded PTFE substrates impregnated with Nafion®. Gore-Select® membranes (available from W.L. Gore & Associates) are composite membranes comprising a microporous expanded PTFE membrane having an ion exchange material impregnated therein.
EP-A 718903 describes a method for making reinforced Proton Exchange Membranes (PEM) by placing the ePTFE on a hoop and impregnating the ePTFE on both sides by placing the ionomer on top of the ePTFE. Both dipping, brushing and spraying of the ionomer are described.
In U.S. Pat. No. 5,547,551 the same principle for the fabrication of the impregnated membrane is described.
U.S. Pat. No. 6,689,501 discloses an asymmetric composite membrane for use in a fuel cell membrane electrode assembly, the composite membrane comprising (a) a porous polymeric substrate; (b) an impregnant comprising a cation exchange material, the impregnant partially filling the substrate such that the substrate comprises a first region having pores substantially filled with the impregnant, and a second substantially porous region; and (c) a dense surface layer comprising the cation exchange material, the dense layer contiguous with the first region of the substrate, wherein the substrate has greater than 10% residual porosity, and the composite membrane is substantially gas impermeable and has a substantially porous major surface.
Two methods for preparation of the composite membrane are described. In the first preparation method a layer of Nafion was applied to a surface of a intermediate support. A porous support was then brought into contact with the wet Nafion layer. The porous support was immediately impregnated with nafion and became transparent. The composite membrane was then dried at 60° C. for 5 minutes to remove the solvent by evaporation. The dry composite membrane was removed from the intermediate support.
In the second preparation method the dry composite membrane on the intermediate support are rolled up in a continuous manner on the wind-up station.
As possible coating methods are mentioned: forwarding roll coating, reverse roll coating, gravure coating, kiss coating, doctor blade coating or die coating.
EP-A 1702668 discloses a composite ionomeric membrane comprising a layer or film of a porous inert support on which a sulphonic (per)fluorinated ionomer is deposited. The membranes are prepared by a process comprising the following steps:
1) preparing a liquid dispersion comprising the (per)fluorinated ionomer, in acid or salified form;
2) depositing the dispersion on the surfaces of a porous support to form a film or layer, thereby obtaining a film of impregnated porous inert material;
3) applying on one side of the film or layer of the impregnated porous inert material a support of a non porous material having smooth surfaces, wherein the support is inert under the conditions used in step 4) of the process;
4) annealing at temperatures from 130° C. to 280° C.;
5) detachment of the membrane from the support.
It is described to apply the ionomer to the ePTFE in order to impregnate it, afterwards the impregnated ePTFE is laid upon an intermediate support followed by drying and annealing. At the end the membrane is delaminated from the intermediate support.
While current composite ion exchange membranes developed for use in fuel cells have achieved a measure of success, there are still many areas for additional improvement. First, as noted above, the microporous substrate is filled with ion exchange material. Generally speaking, the ion exchange material is the most expensive component of the composite. Thus, essentially the impact of the ionomer on the membrane cost should be minimized/optimized as much as possible. The amount of ionomer should be as small as possible. The variation in amount should also be as small as possible. Second, current methods for producing such composite ion exchange membranes typically involve multiple coating steps to fully impregnate the substrate with ion exchange material. Alternatively, or in addition, such methods comprise steps for facilitating impregnation, such as ultrasonication, or adding surfactants to the impregnation solution. These steps increase the time, complexity, and cost of producing composite ion exchange membranes. This is particularly the case where surfactants are added to the impregnation solution, which generally necessitates an additional processing step to remove the surfactant before using the composite membrane in a fuel cell. Third, current methods for producing such composite ion exchange membranes typically have to deal with wetted ePTFE that is very difficult to handle. In some patents it has been described to place the wetted ePTFE upon an intermediate support for easier handling, but the wetting process remains a difficult one. Some patent have described to make laminates of ePTFE with intermediate supports, but even then overcoating of this laminate with a precise metered amount of ionomer is very difficult.
It is desirable to have a composite ion exchange membrane suitable for use in fuel cells that is less expensive and easier to produce than current composite ion exchange and that provide comparable fuel cell performance and improved membrane layer characteristics.